Referring to FIG. 1, an EUV (extreme ultraviolet) lithography system 100 is used for patterning an IC (integrated circuit) material formed on a semiconductor substrate 102. The lithography system 100 includes a light source 104 that generates incident light 106 toward a reticle 108 clamped to a chuck 109. Portions of incident light 106 are reflected by the reticle 108 as reflected light 110 according to a pattern of the reticle 108. The reflected light 110 is projected through an image reduction system 112 onto the semiconductor substrate 102 for patterning the IC material thereon. Such components of the EUV lithography system 100 are known to one of ordinary skill in the art.
The reticle 108 may disadvantageously be clamped to the chuck 109 with a warped shape. For example in FIG. 1, the reticle 108 is clamped to the chuck 109 with a bowing out at the center. With such a warped shape, the incident light 106 is reflected from a bowing surface 114 of the reticle 108. However, such a warped shape contributes to an image placement error. Referring to FIG. 1, if the reticle 108 were clamped flat to the chuck 109, the incident light 106 would be reflected from a flat surface 116 (as illustrated by the dashed line in FIG. 1) to form another reflected light 118 (as illustrated by the dashed line in FIG. 1).
Referring to FIG. 1, the displacement between the reflected lights 110 and 118 forms an image placement error 120, ΔX. In addition, ΔZ is a peak to valley distance 122 which is a maximum distance between the bowed surface 114 and the theoretically flat surface 116 of the reticle. Furthermore, in the example of FIG. 1, the image reduction system 112 reduces the image size by 4:1, and θ° is an angle of incidence of the light 106 from the light source 104. In that case, the image placement error 120, ΔX, is expressed as follows:ΔX=(¼)*tan(θ°)*ΔZ 
In addition, the curvature of the bowed surface 114 may further contribute to the image placement error 120, ΔX. To minimize the image placement error 120, ΔX, the reticle is desired to be flattened against the chuck 109 to minimize ΔZ and to minimize the curvature of the surface of the reticle 108.